1. Field of the Invention
This invention relates to a method for defluorinating wet process phosphoric acid. More particularly, this invention relates to a method for defluorinating wet process phosphoric acid using phosphatic waste clay from the beneficiation of phosphate rock.
2. Description of Related Art
Wet process phosphoric acid, such as that obtained from phosphate rock or ore deposits in central Florida, contains fluorine, e.g., in fluorine compounds, in amounts sufficient to make it unsuitable for use in animal feed additives, human food additives, or as a raw material for the preparation of phosphate compounds used in detergent formulations. In addition, the presence of fluorine in the wet process phosphoric acid causes post-precipitation of complex fluorides which hamper production of merchant acid grades of phosphoric acid, i.e., concentrated acids. Environmental considerations also may require the removal of fluorine from phosphoric acid.
Various methods of defluorination have been proposed. Precipitation with salts of insoluble fluorine-containing compounds is not very useful, because substantial quantities of phosphoric anhydride also are precipitated. These precipitates represent a significant economic loss of phosphate values. Vacuum concentration has been tried, but does not remove enough fluorine to warrant the expense.
A more widely practiced process for removing fluorine from wet process phosphoric acid comprises the addition of silica, such as diatomaceous earth, to concentrated 54 percent P.sub.2 O.sub.5 acid. Diatomaceous earth (tripolite, kieselguhr, for example) is formed from the siliceous shells of diatoms and is found in the beds of ancient seas in such places as the Great Basin of Nevada. The material is also used for filtering, as an adsorbent, as an insulating material, and as an abrasive in soaps. These competing uses have a significant effect on the cost of diatomaceous earth. The cost also is increased by the cost of transporting it to phosphate rock mine sites.
One such process is disclosed in U.S. Pat. No. 2,987,376. A slurry of phosphoric acid and between about 0.3 and 1.2 wt. percent colloidal silica having a specific surface area of at least about 25 square meters per gram is boiled to cause fluorine to be evolved. However, as disclosed in the patent, this method of defluorination is not satisfactory if the acid is dilute, i.e., has a P.sub.2 O.sub.5 concentration below about 30 wt. percent, because defluorination by boiling is slow at these low concentrations. Dilute acid must be concentrated, typically by boiling off water, to between about 50 and 60 wt. percent P.sub.2 O.sub.5, at which concentration the method becomes useful.
U.S. Pat. No. 4,046,860 discloses a method of defluorinating and concentrating wet process phosphoric acid which comprises the addition of about 2 percent by weight colloidal silica to a fluorine-containing phosphoric acid (28-32 percent P.sub.2 O.sub.5), heating under vacuum (50-180 mm Hg absolute) to a temperature of about 190.degree. F., removing the concentrated phosphoric acid stream low in fluorine content, and separately removing a vapor stream containing silicon tetrafluoride (SiF.sub.4). The silicon tetrafluoride then is scrubbed with a solution containing ammonium bifluoride or ammonium fluoride, and optionally, fluosilicic acid or ammonium fluosilicate. The aqueous solution discharged from the scrubber is neutralized with ammonia to precipitate silica and to convert all the fluorine-containing compounds to ammonium fluoride. The silica is washed and recycled.
As described in U.S. Pat. No. 3,800,029, sodium fluosilicate can be recovered from wet process phosphoric acid by adding a sodium compound, such as caustic soda, or by adding a sodium salt to phosphoric acid which does not contain substantial amounts of fluosilicic acid to form a solution containing phosphoric acid and sodium dihydrogen phosphate. This aqueous solution then is added to wet process phosphoric acid containing fluosilicic acid, and the resulting sodium fluosilicate crystals are separated from the defluorinated phosphoric acid.
Fluoride-free phosphoric acid also is said to be produced by the method described in U.S. Pat. No. 4,055,626. Acidulation of phosphate rock with sulfuric acid is conducted in the presence of added potassium (e.g., KHSO.sub.4, K.sub.2 SO.sub.4, KH.sub.2 PO.sub.4, or KOH) and additional silica to precipitate fluorides and silica as potassium silicofluoride.
French Patent Application No. 76-05359 discloses a process for simultaneous clarification and defluorination of phosphoric acid solution wherein finely divided bleaching earth is mixed with the acid solution, agitated for a period sufficient to ensure reaction of fluoride in phosphoric acid with the silica in the bleaching earth and the adsorption of organic substituents in the acid onto the earth particles. Typically, up to about 30 minutes agitation is allowed. Suitable bleaching earths include colloidal clays (smectic clay, bentonites, and polygorskites) and clays having a high proportion of siliceous material, such as montmorillonite or attapulgite.
Clarified and defluorinated phosphoric acid is separated from the bleaching earth by filtration or by decantation over a period of between about three and ten days. Higher temperatures promote defluorination, so, typically, the temperature is maintained between about 50.degree. and 90.degree. C. Five to 30 grams of clay is utilized per kilogram of acid treated to ensure effective defluorination without wasting defluorinating agent. In treated acid, a typical carbon concentration is less than about 0.20 wt. percent and fluorine content is less than about 0.25 wt. percent after a treatment of about three days.
French Application for Certificate of Addition 81-11229 discloses that practice of the method of FR No. 76-05359 with hydrated clay reduces the quantity of clay required by a factor of about 3 and reduces the period required for treatment to a minimum of about 24 hours.
These methods are unsatisfactory, however, for two reasons. First, they require the use of bleaching earths, which suffer the same disabilities as diatomaceous earth. Further, a period of at least about 24 hours is required to obtain the described fluorine concentration.
In the method described in U.S. Pat. No. 4,330,517, fumes (dust) from submerged arc furnaces used to make silicon alloys with metals such as iron and chrome are utilized in place of, for example, diatomaceous earth, to defluorinate wet process phosphoric acid. These fumes contain silica in fine particulate form, and are quick, effective defluorinating agents. Unfortunately, these fumes originate from submerged arc furnaces of the type described above, which are not generally located in the vicinity of the wet process acid facility. Therefore, these fumes typically must be transported to the site where wet process acid is defluorinated.
U.S. Pat. No. 4,000,067 discloses a method for rapid sedimentation of fine particles from aqueous suspensions of phosphatic waste clays by adding fluoride. In particular, the patent discloses a method for reacting, under acidic conditions, a fluoride ion-containing material with phosphatic waste clays from phosphate rock beneficiation to rapidly settle particles. Phosphatic waste clays, said to be suitably treated by the method, include those suspensions which contain siliceous materials, such as attapulgite, montmorillonite, and kaolinite clays. Suitable fluoride iron-containing materials have a pH between about 0.5 and 5, and hydrofluoric acid is preferred.